What do both families do to mend their differences in the end? Is there irony in this?

Answers
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Both Capulet and Montague promise to erect golden statues of each other's child. That's pretty much it. The irony is that their children are dead and these statues will somehow make up for their ignorance. It all seems an extension of their own values; their money can somehow fix their part in the death of R&J.